The Particle Whisperers

As many parents know, it’s often easier to keep your kids under control by exerting less authority rather than more. A child who fidgets uncontrollably in a confining booster seat, for example, may be perfectly content on a plain old chair. A team of Spanish physicists has found that the same is true in controlling the movement of particles suspended in liquids. What’s more, they speculate that many microscopic systems, macroscopic ecosystems, and human social systems may respond to a gentle touch for the very same mathematical reasons.

In order to test their hypothesis that heavy handedness can lead to loss of control, the researchers used optical tweezers to grab hold of floating microscopic beads. They then dragged the particles back and forth in the fluid as they ramped up the intensity of the lasers that formed the tweezers. As they expected, increasing laser power provided an ever tighter grip on captured particles, but only up to a point. Eventually, ramping up the laser led to a poorer control of the particles, which jostled around more and more as the laser intensity increased.

The experiment was consistent with a simple mathematical model that the researchers suggest could be helpful in optimizing all sorts of systems, from high-resolution microscopes to managed ecosystems such as national parks. It could even help mathematically explain why overly strict social policies may lead to chaos and revolution, and how iron-fisted fiscal policies can potentially drive economic systems to ruin. -JR

Quantum Mechanical Con Game

N. Aharon and L. VaidmanPhysical Review A (forthcoming)

For the first time, physicists have come up with a scheme in which a quantum mechanical expert never loses in a con game with a victim who only knows about classical physics. In prior quantum cons the real actions of the classical players usually eliminate the supposed advantage of the quantum player.

A pair of physicists at Tel-Aviv University in Israel came up with the quantum game by imagining two people betting on the location of a particle hidden in one of two boxes. In the game, a quantum mechanical con named Alice prepares a particle in a system of two boxes or any other place other than these two boxes. She then turns away as her classical victim, Bob, is allowed to look inside one of the two boxes sitting on a table to see if there is a particle inside. He then closes the box he looked in and Alice gains access to the two boxes. She is allowed to make any measurement she wants, after which she must declare whether she wants to play this round of the game or not. If she plays, she wins if Bob found the particle.

Classically, there is a 50% chance of Alice getting it right. If instead she’s adept at quantum mechanics, and has a third box hidden away, she can ensure that she never approves a trail in which Bob did not find the particle. All she has to do is prepare the particle in a state that essentially places the particle in all three boxes simultaneously, using a phenomenon known as quantum superposition. In effect, there is an equal chance of the particle turning up in any of the three boxes. After Bob looks in one of the two boxes on the table, Alice measures the state of the particle in all three boxes. If she finds that the particle is in a certain superposition state, she knows Bob saw the particle in the box he opened, even though she has no way of knowing in advance where the particle will turn up or which box Bob will choose to look in. If she does not find that the particle is in the desired superposition state, she knows that there is a chance that Bob opened a box but didn’t see anything inside. In this case, she passes on this round of the game.

The authors of the paper admit that the current state of technology isn’t good enough for a con make money with quantum mechanics. But they believe that this is the first time anyone has shown that it’s theoretically possible for someone like Alice to use quantum mechanics in order to ensure that she never loses (and sometimes wins) in a game that classical physics would only give her a fifty-fifty chance of winning. With advances in quantum technology, it may someday turn out that gambling is only risky for those of us who don’t understand quantum mechanics.

Kenneth Wilson received the 1982 Nobel Prize in physics for his theory of phase transitions based on what is called the renormalization group. He built on previous work by Leo Kadanoff and Michael Fisher, with whom he shared the 1980 Wolf Prize. Wilson’s Nobel Prize winning work was published in 1971 in Physical Review B [PRB 4, 3174, and PRB 4, 3184]. In the Letters quoted here several consequences of the work in the PRB publications were developed and were used in much future research. An excellent, accessible summary of the background and content of Wilson’s research and of its impact is available in the Nobel committee’s 1982 press release.

Osheroff, Richardson, and Lee were looking for a phase transition to a kind of magnetic order in solid helium-3 at very low temperatures. They were achieving these temperatures by using what was called “Pomeranchuk cooling” - putting pressure on a mixture of solid and liquid helium-3. In the first Letter two anomalies in the measurement were interpreted as phase transitions in solid helium-3. But because of uncertainties in the interpretation the authors used a crude version of what later was named “magnetic resonance imaging” to determine that the anomalies were in the liquid. Other measurements led to the conclusion that they had in fact discovered superfluid helium-3. An important element in this interpretation was the theoretical work of Leggett (see following Milestone). For this research Osheroff, Richardson, and Lee received the 1996 Nobel Prize in Physics “for the discovery of superfluid helium-3”. For further information see David Lee’s Nobel lecture [Rev. Mod. Phys. 69, 646 (1997)] and the Nobel Committee’s press release.

Leggett describes in his Nobel lecture how he interrupted a holiday (because it rained on that day!) to meet with Robert Richardson at Leggett’s office in Sussex. Richardson described the experimental results on phase transitions in liquid He3, which set Leggett into deep thought on their theoretical explanation. This Letter presents the foundation for understanding the properties of these complex superfluid phases. Many others contributed to that understanding as well, but Leggett was credited with the complete theoretical underpinning. A. A. Abrikosov, V. L. Ginzburg, and A. J. Leggett were awarded the 2003 Nobel Prize in Physics “for pioneering contributions to the theory of superconductors and superfluids”. See also Leggett’s Nobel Lecture [Rev. Mod. Phys. 76, 999 (2004)], and the Nobel Focus story Phys. Rev. Focus 12, story 16.

James Riordon contributed to this Tip Sheet.

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